1. Field of the Invention
The present invention relates to a carbon fiber having a high strength and a high elastic modulus, which is prepared from an optically anisotropic pitch as the starting material, and a process for the preparation of this carbon fiber. More particularly, the present invention relates to a pitch-based carbon fiber having a microstructure consisting of strip-like structural units, in which the strength and elastic modulus are highly improved because the configuration of the strip-like structural units in the cross-section of the fiber takes a fractal structure, and a process for preparing the fiber on an industrial scale.
2. Description of the Related Art
Initially, carbon fibers were prepared by using rayon as the starting material, but at present, carbon fibers are occupied by PAN carbon fibers prepared from polyacrylonitrile (PAN) fibers as the starting material and pitch-based carbon fibers prepared from coal or petroleum pitch as the starting material from the viewpoints of characteristics and the economical viewpoint. Especially, the technique of preparing a carbon fiber having a high performance grade from pitch as the starting material attracts attention because this technique is excellent from the economic viewpoint. For example, it is known that a carbon fiber obtained by melt-spinning an optically anisotropic pitch, rendering the spun fiber infusible and carbonizing the fiber has higher strength and higher elastic modulus than those of conventional pitch-based carbon fibers (see Japanese Examined Patent Publication No. 54-1810 or U.K. Patent No. 1,496,678).
However, in case of pitch-based carbon fibers, cracks are formed along the direction of the fiber axis at the preparation steps, and even if cracks are not formed, the fibers are very brittle, and it is difficult to obtain a carbon fiber having improved strength and elastic modulus.
Under this background, trials have been made to improve the physical properties of carbon fibers by controlling the cross-sectional structure of the fibers. The cross-sectional structure heretofore discussed is the selective orientation state on the carbon layer face presumed by observation of the cross-section of the fiber just after melt spinning or after carbonization or graphitization by a polarization microscope or scanning electron microscope. In general, the structure in which carbon layer faces are radially arranged in the cross-section of the fiber is called "the radial structure", the structure in which carbon layer faces are concentrically arranged is called "the onion structure", and the structure in which the selective orientation is obscure is called "the random structure".
It is known that of these structures, it is the radial structure that causes cracking, and therefore, the preparation technique of manifesting a cross-sectional structure other than the radial structure has been vigorously sought for.
For example, Japanese Unexamined Patent Publication No. 59-53717, Japanese Unexamined Patent Publication No. 59-76925, Japanese Unexamined Patent Publication No. 59-168127 propose the onion or random structure, Japanese Unexamined Patent Publication No. 59-163424, proposes the random structure, and Japanese Unexamined Patent Publication No. 59-163423 proposes the distorted radial structure or the random structure. Each of these structures is formed by adopting specific spinning conditions or using a spinning nozzle having a specific shape. Furthermore, Japanese Unexamined Patent Publication No. 61-186520 and Japanese Unexamined Patent Publication No. 61-12919 teach that a cross-sectional structure other than the radical structure is formed by placing a filler just above the spinning nozzle, and Japanese Unexamined Patent Publication No. 62-177222 and Japanese 63-75119 teach that a cross-sectional structure other than the radial structure is formed by arranging a stationary or dynamic stirring apparatus on the spinning nozzle.
However, these processes commonly involve the following two problems.
(1) The reproducibility of manifestation of the desired cross-sectional structure is poor, and prevention of formation of cracks is not complete.
(2) Even if the desired cross-sectional structure is manifested and cracks are not formed along the fiber axis, the brittleness of the fiber is not eliminated.
The technique of solving these problems and stably providing a high-strength pitch-based carbon fiber having a strength of above 400 kg/mm.sup.2, that is, a high strength comparable to that of the PAN type carbon fiber was not completed.
As the means for solving these problems effectively, Sasaki et al proposes in U.S. Pat. No. 4,628,001 a process in which a leafy structure is formed by using a non-circular spinning nozzle having a specific shape. According to this process, formation of cracks along the direction of the fiber axis can be completely prevented and a tensile strength exceeding 400 kg/mm.sup.2 is realized. Furthermore, Japanese Unexamined Patent Publication No. 61-113827 proposes a spinning process using a non-circular spinning nozzle having a specific shape, in which a dividing pitch flow path controlling element is arranged on the nozzle. Even according to these processes, however, the strength of the obtained carbon fiber tends to decrease with increase of the Young's modulus, and it is difficult to maintain a tensile strength exceeding 500 kg/mm.sup.2 when the Young's modulus is higher than 30 T/mm.sup.2. Moreover, even in the case where increase of the Young's modulus is not especially aimed, the problem of the low elongation considered to be an inherent problem of carbon fibers is not solved, and a carbon fiber having a strength exceeding 500 kg/cm.sup.2 and simultaneously, an elongation higher than 2.5% cannot be realized. Furthermore, the pitch-based carbon fiber prepared according to this process has inevitably a non-circular cross-section, and the process is defective in that an optional cross-sectional shape cannot be selected.